Conventionally, low-frequency filters have been manufactured passive also. Their disadvantages include in particular the non-idealness caused by coils, the large size and relatively high production costs. Active filters realized using discrete components do not have the disadvantages caused by coils, but because of the number of components they, too, are space-consuming and have relatively high production costs.
An active filter can be realized in a small space by integrating it to a microcircuit. The problem with such filters is the large area required by the capacitances on the chip. Integration is possible if the capacitances are made very small and correspondingly the resistances very big. This means, however, that the signal level will drop and the noise level will increase and, therefore, this solution is usually unacceptable. Integration is possible also if small capacitances are used with very high virtual resistances based on the switched capacitor (SC) or switched current (SI) technology, for example. This eliminates high thermal noise levels, but the use of switches will result in the increase of noise level, increased current consumption and deterioration of the linearity of the filter. The latter will limit the dynamic range of the filter. If the apparatus in question is a radio device, the use of switches may also cause interference problems in the RF circuits of the apparatus. The filter may also be made such that the parts that are difficult to integrate are left outside the micro circuit. A disadvantage of such a construction is that calibration becomes more difficult; The filter requirements are usually so strict that, regardless of the construction, calibration is necessary because of the variation in component values. In the mixed construction mentioned above the deviations of the values of discrete and integrated components do not correlate, which means the calibration of filters in production may cause higher costs than totally discrete or totally integrated filters.